(1) Field of the Invention
This invention relates to a controlling method and device for controlling driving of a permanent magnet synchronous motor wherein a permanent magnet is used as a rotor.
(2) Description of the Prior Art
A permanent magnet synchronous motor is a motor of such a construction that a permanent magnet is used as a rotor and ac currents are supplied to armature windings to form a rotating magnetic field in order to cause the rotor to rotate. General construction of a permanent magnet synchronous motor of such a construction and a controlling device for the motor will be described with reference to the drawings.
FIG. 7 is a circuit diagram of a conventional controlling device for a permanent magnet synchronous motor. Referring to FIG. 7, a permanent magnet synchronous motor M is composed of a permanent magnet 1 and three armature windings 2U, 2V and 2W. Three-phase ac currents are supplied to the armature windings 2U, 2V and 2W. In particular, electric currents I.sub.U, I.sub.V and I.sub.W of the U, V and W phases are supplied to the armature windings 2U, 2V and 2W, respectively. A three-phase ac power source 3 is provided for driving the permanent magnet synchronous motor M. A rectifier circuit 4 constituted from six transistors Tr rectifies three-phase ac currents of the power source 3 into dc currents. A rectifier controlling circuit 5 develops a controlling signal for each of the transistors Tr, and in response to controlling signals from the rectifier controlling circuit 5, appropriate rectification of the three-phase ac currents is effected. An invertor circuit 6 composed of six parallel circuits each including a transistor Tr and a diode D converts dc currents rectified by the rectifier circuit 4 into three-phase ac currents. An invertor controlling circuit 7 develops a controlling signal for each of the transistors Tr of the invertor circuit 6. A circuit consisting of the power source 3, rectifier circuit 4, rectifier controlling circuit 5, invertor circuit 6 and invertor controlling circuit 7 will be hereinafter referred to as an invertor device which is generally denoted by a reference numeral 8.
With the controlling device for a permanent magnet synchronous motor described above, if predetermined controlling signals are developed from the rectifier controlling circuit 5 and the invertor controlling circuit 7, corresponding three-phase currents are supplied to the armature windings 2U, 2V and 2W, and a rotating magnetic field is formed by the currents of the individual phases. As the rotating magnetic field is generated, the permanent magnet 1 is rotated by and in synchronism with the rotating magnetic field.
Such a permanent magnet synchronous motor as described above has significant advantages that, since a permanent magnet is employed used as a rotor, it requires no exciting power and accordingly the power consumption is remarkably low comparing with some other motor of the same capacity such as, for example, an induction motor, and that, since it requires no slip ring nor brush, it seldom suffers from a trouble or loss.
While a permanent magnet synchronous motor has such advantages as described just above, it has following problems. In particular, it is a common case that, as the time of use passes, a permanent magnet loses its magnetomotive force and is demagnetized gradually. This will be described with reference to characteristic curves of permanent magnets shown in FIG. 8. Referring to FIG. 8, the axis of abscissa represents coercive force while the axis of ordinate represents magnetic flux density. A permanent magnet used in a permanent magnet synchronous motor normally has a magnetic flux density as high as possible so that it may not lead to shortage of a turning effect. However, a permanent magnet which has a high magnetic flux density naturally has a small coercive force. In particular, referring to FIG. 8, a permanent magnet having a magnetic flux density B.sub.m1 has a coercive force H.sub.c1, but the coercive force H.sub.c2 of another permanent magnet having a magnetic flux density B.sub.m2 lower than the magnetic flux density B.sub.m1 is greater than the coercive force H.sub.c1. From this reason, a permanent magnet having a high magnetic flux density must be used for such an apparatus as a permanent magnet synchronous motor for which a considerably high turning effect is required. Besides, since such a permanent magnet presents a small coercive force, demagnetization thereof cannot be avoided after all.
Further, demagnetization is caused not only by such aging. During driving of a motor, such an instant frequently occurs that a north pole and a south pole of a permanent magnet are opposed to a north pole and a south pole, respectively, of a magnetic field generated by armature windings 2U, 2V and 2W, which also will cause demagnetization of the permanent magnet.
Meanwhile, if a heavy load is applied to the permanent magnet synchronous motor, the permanent magnet synchronous motor may be stepped out. In such a case, if such a condition takes place that rotation of the permanent magnet is stopped due to the relation to the load then, such a positional relationship between the magnetic poles of the permanent magnet and the magnetic field by the armature windings as described above may be maintained for a long period of time. Consequently, the permanent magnet may be demagnetized to a significant degree, and in an extreme case, there is the possibility that the permanent magnet may be completely demagnetized.
In this manner, a permanent magnet synchronous motor cannot be free from partial or complete demagnetization, and such partial or complete demagnetization will give rise to shortage or disappearance of a turning effect of the permanent magnet synchronous motor. This may lead to a problem that the permanent magnet synchronous motor cannot rotate itself at all. This problem naturally depreciates the reliability of a permanent magnet synchronous motor to a remarkable degree. Accordingly, a permanent magnet synchronous motor which lacks in reliability as described just above cannot at all be employed in various equipments of various fields, particularly an equipment of a field for transporting human beings such as an elevator or an escalator while a permanent magnet synchronous motor has such advantages as described hereinabove.